Tabs are bookmarks that can keep their state. Normal bookmarks have only the URL, and any parameters that go in the URL, but discard the rest, like your position on the page, your navigation history, or what you are doing in a PWA.

Tabs don't discard data, bookmarks do. Sometimes you just want the landing page and a bookmark is fine, other times you want the extra data and a long-lived tab is needed.

Many tabs refresh after no activity

Tabs and bookmarks still feel pretty different. To update a tab, you activate the tab and then click links as usual. To update a bookmark, you activate the bookmark, click links, delete the old bookmark, create a new bookmark, and drag the new bookmark to the old one's location in the bookmark list (unless I've missed a shortcut here). Tabs feel very dynamic and are useful as a list or queue of work-in-progress, and bookmarks feel very static and are useful as a permanent list of important resources.

Look at the new Firefox Preview on Android. They're embraced that way.